A new technique for detecting oncogene activation based on immunoblotting for oncogene proteins in serum has been applied to screen a cohort of foundry workers with well-defined workplace exposures to polycyclic aromatic hydrocarbon carcinogens. Three of the 18 individuals screened were found to have abnormal expression of the proteins of the ras and fes oncogenes. These three individuals were known to have had medium to high workplace exposures to benzo(a)pyrene and to have correspondingly high levels of benzo(a)pyrene-DNA adducts in their peripheral leukocytes. No individuals among the unexposed controls were found to have abnormal serum oncogene protein expression. These results suggest the feasibility of using serum oncogene proteins along with DNA-carcinogen adducts as potential molecular epidemiological markers in exposed worker populations; further, larger scale studies will be necessary to demonstrate the utility of these markers for identifying individuals at risk for the development of malignant disease due to their occupational exposures.